Bile pigments in algae and mammals are synthesized from heme via a Two-Molecule Mechanism in which the lactam oxygen atoms on the external rings arise from molecular oxygen, but from different oxygen molecules. This process will be investigated by measuring incorporation of 18,1802 into the algal bile pigment, phycoerythrobilin. Bile pigments in plants are covalently attached to apoproteins which aggregate into multimeric light harvesting "organelles", called phycobilisomes. The primary structure of the apoproteins from phycocyanin isolated from the alga, Cyanidium caldarium, will be completed by automated Edman degradation of certain peptides. The apoproteins to which algal bile pigments are covalently attached can constitute up to 60% of the total soluble protein in cells and studies are in progress in which the mRNAs for apoproteins are translated in a reticulocyte cell free system. These studies are being performed to determine certain of the events in regulation of protein and bile pigment synthesis which occur during phycobilisome formation in red algae and cyanobacteria.